This invention relates to a cast steel strip produced in a strip caster, particularly a twin roll caster.
In a twin roll caster, molten metal is introduced between a pair of contra-rotated horizontal casting rolls, which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a cast strip product delivered downwardly from the nip between the rolls. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel from which it flows through a metal delivery nozzle located above the nip. The molten melt forms a casting pool supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to restrain the two ends of the casting pool against outflow, although alternative means such as electromagnetic barriers have also been proposed.
When casting steel strip in a twin roll caster the strip leaves the nip at very high temperatures of the order of 1400° C., or higher, and if exposed to air, exiting cast strip suffers very rapid scaling due to oxidation at such high temperatures.
It has therefore been proposed to shroud the newly cast strip within an enclosure containing a non-oxidizing atmosphere until its temperature has been reduced significantly, typically to a temperature of the order of 1200° C. or less so as to reduce scaling. One such proposal is described in U.S. Pat. No. 5,762,126 according to which the cast strip is passed through a sealed enclosure from which oxygen is extracted by initial oxidation of the strip passing through it. Thereafter, the oxygen content in the sealed enclosure is maintained at less than the surrounding atmosphere by continuing oxidation of the strip passing through it, so as to control the thickness of the scale on the strip emerging from the enclosure. The emerging strip is reduced in thickness in an inline rolling mill and then generally subjected to forced cooling, for example by water sprays, and the cooled strip is then coiled in a conventional coiler typically in 20-ton coils.
Previously, it has been proposed in strip casting to cool the strip through the austenite transformation zone by subjecting the strip to water sprays. Such water sprays are capable of producing maximum cooling rates of the order of 90° C./sec. The degree to which cooling can be used to control cooling rates can be used to control the microstructure of the cast strip as illustrated by U.S. Pat. No. 6,328,826, where cooling rates between 5° C. and 100° C./sec. produce Transformation Induced Plasticity (TRIP) steel with a microstructure of at least 5% austenite and both high strength and high ductility properties suitable for shaping.
Previously, it has been proposed in strip casting to cool the strip to thin steel sheet with excellent stretchability by cooling said thin cast strip from the temperature range of from the casting temperature to 900° C. to a temperature of not higher than 650° C. at an average cooling rate of not less than V (° C./sec) represented by the following formula; and coiling the cooled strip at a temperature of not more than 650° C.:log V≦0.5−0.8 log Ceq(° C./sec)wherein Ceq=C+0.2 Mn. See U.S. Pat. No. 5,567,250. This cooling regime provided a thin cast strip with a microstructure selected from a transgranular acicular ferrite and/or a bainite having a packet size of 30 to 300 μm in a proportion of not less than 95% of the structure. Thus, according to the previous teaching, a low-temperature transformation phase advantageous for the stretch-flange ability can be wholly provided by causing transformation at a certain or higher cooling rate which does not form coarse ferrite. Col. 6, II. 17–28.
According to the present disclosure, a cast steel strip is prepared for example by a process comprising the steps of:
continuously casting molten plain carbon steel into a strip of not more than 5 mm in thickness and including austenite grains;
passing the strip through a roll mill in which the strip is hot rolled to produce a reduction in strip thickness by more than 15%; and
cooling the strip to transform the strip austenite to ferrite within the temperature range of between 400° C. to 850° C. at a cooling rate of more than 100° C./sec to form cast strip that is less than about 1% austenite and has a packet size of at least 10% greater than 300 μm, is either (i) a mixture of polygonal ferrite and low temperature transformation products or (ii) predominantly low temperature transformation products, and has a yield strength of at least 450 MPa.
The cast steel strip may be prepared by a process comprising the steps of:
continuously casting molten plain carbon steel into a strip of not more than 5 mm in thickness and including austenite grains;
passing the strip through a roll mill in which the strip is hot rolled to produce a reduction in strip thickness by more than 15%; and
continuously cooling the strip to transform the strip austenite to ferrite within the temperature range of between 400° C. to 850° C. at a cooling rate of greater than 100° C./sec without inhibiting the cooling rate to form cast strip that is less than about 1% austenite and has a packet size of at least 10% greater than 300 μm, is either (i) a mixture of polygonal ferrite and low temperature transformation products or (ii) predominantly low temperature transformation products, and has a yield strength of at least 450 MPa.
In the described processes used to produce the cast steel strip, the strip is continuously cast by supporting a casting pool of molten steel on a pair of chilled casting rolls forming a nip between them, and producing cast strip by counter-rotating the casting rolls in opposite directions such that the casted strip moves downwardly from the nip.
In both of the described processes, the cooling step may start at least 10° C. above the Ar3 temperature. The cooling step may start at 800° C. or above. The cooling rate may be in the range from greater than 100° C./sec to 300° C./sec. The strip may be cooled through the transformation temperature range within between 400° C. and 850° C., and not necessarily through that entire temperature range at such a cooling rate. The precise transformation temperature range will vary with the chemistry of the steel composition and processing characteristics.
We have found that it is possible to achieve a remarkable degree of hardenability in typical plain carbon steel chemistry by employing accelerated cooling rates, to promote the formation of low temperature transformation products which enables an increased range of strip products to be produced, particularly with a range of yield strength and hardness, even in the case where inline heat reduction has refined the ‘as cast’ microstructure.
The term “packet size” refers to the grain orientation within a group of grains of the microstructure. Grains have similar orientation within a packet. Packets are identified in micrographs by the grain orientation change in grains between different packets. A packet size with 10% greater than 300 μm refers to the grain size of the original austenite grains.
The term “low carbon steel” is understood to mean steel of the following composition, in weight percent:
C:0.02–0.08Si:0.5 or less;Mn:1.0 or less;residual/incidental impurities:1.0 or less; andFe:balance
The term “residual/incidental impurities” covers levels of elements, such as copper, tin, zinc, nickel, chromium, and molybdenum, that may be present in relatively small amounts, not as a consequence of specific additions of these elements but as a consequence of standard steel making. Elements may be present as a result of using scrap steel to produce plain carbon steel.
The low carbon steel may be silicon/manganese killed and may have the following composition by weight:
Carbon0.02–0.08%Manganese0.30–0.80%Silicon0.10–0.40%Sulfur0.002–0.05%Aluminumless than 0.01%
Silicon/manganese killed steels are particularly suited to twin roll strip casting. A silicon/manganese killed steel will generally have a manganese content of not less than 0.20% (typically about 0.6%) by weight and a silicon content of not less than 0.10% (typically about 0.3%) by weight.
The low carbon steel may be aluminum killed and may have the following composition by weight:
Carbon0.02–0.08%Manganese0.40% maxSilicon0.05% maxSulfur0.002–0.05%Aluminum0.05% maxThe aluminum killed steel may be calcium treated.
The cast steel strip may be produced with a yield strength in the range of 450 MPa to in excess of 700 MPa by cooling rates in the range of greater than 100° C./sec to 300° C./sec. However, the aluminum killed steels will be generally 20 to 50 MPa softer than the silicon/manganese killed steels.
The cast steel strip may be passed from the casting pool through an enclosure containing an atmosphere, which inhibits oxidation of the strip surface and consequent scale formation. The atmosphere in said enclosure may be formed of inert or reducing gases or it may be an atmosphere containing oxygen at a level lower than the atmosphere surrounding the enclosure. The atmosphere in the enclosure may be formed by sealing the enclosure to restrict ingress of oxygen containing atmosphere, causing oxidation of the strip within the enclosure during an initial phase of casting thereby to extract oxygen from the sealed enclosure and to cause the enclosure to have an oxygen content less than the atmosphere surrounding the enclosure, and thereafter maintaining the oxygen content in the sealed enclosure at less than that of the surrounding atmosphere by continuous oxidation of the strip passing through the sealed enclosure thereby to control the thickness of the resulting scale on the strip.
The strip may be passed through a rolling mill in which it is hot rolled with a reduction in thickness of up to 50%.
Illustratively, the cast strip passes on to a run-out table with cooling means operable to cool the cast strip transforming the strip from austenite to ferrite in a temperature range of 400° C. to 850° C. at a cooling rate greater than 100° C./sec to form cast strip that is less than about 1% austenite and has a packet size of at least 10% greater than 300 μm, is either (i) a mixture of polygonal ferrite and low temperature transformation products or (ii) predominantly low temperature transformation products, and has a yield strength of at least 450 MPa.
The term “low temperature transformation products” includes Widenmanstatten ferrite, acicular ferrite, bainite and martinsite.